This invention relates generally to a control method for a multi-component slurrying process at an oil or gas well.
A "cementitious slurry" as the term is used in this disclosure and in the accompanying claims encompasses mixtures that are made at an oil or gas well in a fluid state so that they can be pumped into the well but which ultimately harden in the well to provide sealing and compressive strength properties useful for known purposes in the well. For example, a settable mud is one type of cementitious slurry, and a cement is another type of cementitious slurry.
When a cementitious slurry is needed, a qualified person analyzes the particular situation and designs a particular slurry. Such a design includes a list of ingredients (the "recipe") and possibly one or more desired parameters (e.g., density). Such a design has at least one of what is referred to herein as a "defining characteristic". For a settable mud, a defining characteristic is the recipe of ingredients. For a cement, a defining characteristic is density.
The design is implemented at the well by mixing the ingredients in a manner to obtain the one or more defining characteristics. The ingredients that are mixed can be of two types: essential materials and additives. As used in this description and the accompanying claims defining the present invention, "essential materials" are ingredients that are required to obtain a particular defining characteristic of a slurry; "additives" are ingredients that modify or enhance the defining or other characteristics of the slurry. Any particular slurry will always have essential materials, but it may or may not have additives.
For the slurries and fluids to which the overall process disclosed herein is directed, there are always at least three essential materials for obtaining a defining characteristic. For example, a defining characteristic of a cement slurry is density; three essential materials for obtaining this characteristic are a hydrating fluid (e.g., fresh water, seawater, brine), a cementitious substance (e.g., cement), and a density control agent (e.g., fly ash). As a further example, a defining characteristic of a drilling fluid is also density; three essential materials for obtaining a desired density in a drilling fluid are a fluid medium (e.g., fresh water, seawater, brine, hydrocarbon fluid), a viscosity control agent (e.g., bentonite), and a density control agent (e.g., barite). As another example, a defining characteristic of a settable mud is the recipe itself; three essential materials for a settable mud recipe are a dilution fluid (e.g., fresh water, seawater, brine, hydrocarbon fluid), a drilling fluid such as referred to above, and a cementitious substance (e.g., cement, fly ash, blast furnace slag).
Although at least three essential materials are needed to obtain a defining characteristic of the type, and for the slurries, referred to herein, slurry mixing processes have typically provided for continuously mixing only two primary flows of essential material. Such limitation necessitates that other essential materials and additives be premixed with one of the two primary flows.
In typical present oil field cementing processes, a single liquid stream and a single dry stream are mixed into the desired cement slurry. An essential material of the liquid stream may be fresh water, for example, and an essential material of the dry stream is cement. When the third essential material is fly ash, for example, and when dry additives, such as retarders and dispersants, are used, they are preblended into the dry cement before continuous two-stream slurrification begins.
A shortcoming of such a preblending process is reduced flexibility in the logistics when cementing in remote locations. For example, offshore locations generally do not have blending facilities; hence, if dry additives are required, they must be blended with the cement at a land-based bulk plant and brought out prior to the job. Lack of homogeneity in the preblended dry materials is another shortcoming of this process because of potential poor performance of the cement downhole. That is, the physical and chemical properties of the cement slurry vary due to the lack of homogeneity and thus do not meet the job design criteria, whereby downhole performance deviations might occur.
Mixing of two flow streams is also used in settable mud systems. Although two essential liquids (drilling fluid and water), an essential dry material (the cementitious substance), and multiple lesser amount substances (dry and liquid additives for activating the cementitious substance and for controlling the slurry properties) may be used to produce a desired settable mud, the current practice is to premix the two essential liquids and all the additives in a large holding volume. A continuous mixing process is then used for adding the single essential dry material stream to a single fluid stream of the premixed substances.
A shortcoming of this two-stream settable mud slurrying process is that it requires space for a large storage facility (e.g., 400-800 barrels) to hold the combined volume of premixed substances prior to performing the two-stream slurrying process. Such a large space is typically not available on an offshore platform or ship; however, there is typically space at offshore locations for storing the individual components separately.
This two-stream settable mud slurrying process has other disadvantages, including: pretreated drilling fluid properties can deteriorate in the holding tanks (for example, adding a dispersant and/or dilution fluid to the drilling fluid causes solids to settle if adequate agitation is not provided, and many drilling rigs do not have adequately agitated pits); and the slurry design and testing must begin several days in advance of the placement downhole so that the drilling fluid can be treated, therefore last minute changes and "on-the-fly" changes cannot be made.
Cementitious slurrying, especially settable mud slurrying just referred to, is the primary context of the overall process disclosed herein. As mentioned above, however, a drilling fluid is typically used as a primary component of a settable mud slurry. A drilling fluid such as is used to flush drilled cuttings from the wellbore is not a cementitious slurry as that term is defined above; however, a drilling fluid is typically made using a principally two-stream process. For example, a fluid medium (e.g., water) can be pumped into a well as an initial drilling fluid. This mixes with downhole materials to form a mixture that flows to the surface where it is retained in a storage facility such as a pit or tank. A further drilling fluid is typically made by flowing a stream of the fluid medium (which may be provided as two streams, such as a water stream and a liquid hydrocarbon stream) and a stream of the mixture from the storage facility into a mixing unit. Control of the defining characteristic of this drilling fluid typically occurs by adding substances into the stream of mixture from the storage facility.
A shortcoming of this drilling fluid process is that the substances added to the mixture stream are input in doses so that correct proportioning does not occur until after mixing in the mixing unit for a sufficient period of time. That is, this prior process does not enable a continuous properly proportioned drilling fluid to be produced and used quickly. As a result, a drilling fluid that may be needed quickly must be made ahead of time and stored at the well site, which can create problems of the type referred to above concerning whether storage space is available and whether homogeneity can be maintained. For example, a relatively heavy drilling fluid referred to as "kill mud" may be required at a well site so that it can be pumped into a well to "kill" it if conditions warrant. With the prior process, kill mud has to made and stored because the prior process cannot continuously produce it with the proper defining characteristic(s) at the time an emergency requiring it arises. This requires the kill mud to be stored somewhere at the well site; this permits changes to occur in the kill mud whereby it may not be suitable when it is needed; and this wastes materials and money and requires disposal procedures if the kill mud is not used.
In view of the foregoing, there is the need for an improved continuous multi-component slurrying process at an oil or gas well, particularly one providing continuous properly proportioned mixing of multiple essential materials and multiple additives to form cementitious slurries or drilling fluids at an oil or gas well site, whether onshore or offshore. That is, such a process should enable slurrying without requiring premixing. Although such a needed process might be manually controlled, it would be preferable to provide an automatic control method for the multi-component slurrying process. It is to this preference for automatic control that the present invention is particularly directed.